1. Field of the invention
This invention relates to a multi-layered dielectric film that is used as a coating film coated on the facets of semiconductor laser devices such as distributed feedback or distributed Bragg reflection semiconductor laser device's with gratings inside thereof, so as to control oscillation operation in the Fabry-Perot resonator of the semiconductor laser devices and to attain oscillation arising from the diffraction of laser due to the inner gratings. This invention also relates to a multi-layered dielectric film that is coated on the end surfaces or other surfaces of optical products such as optical discs used in optical communication systems, so as to improve the optical transmission efficiency thereof and to prevent the ghost phenomena therein.
2. Description of the prior art
As a low-reflection protective film that is coated on the facets of, for example, a semiconductor laser device, a film of Al.sub.2 O.sub.3 or a multi-layered dielectric film that is composed of this film and another dielectric film has been used, which is disclosed, for example, in Japanese Laid-Open Patent Publication No. 61-207091.
This film made of Al.sub.2 O.sub.3 with a thickness of 1/4 of the target wavelength that is much used as a dielectric coating film with a low reflection has a theoretical reflectivity of 0 for the specific wavelength, but the reflectivity thereof at wavelengths nearby the said specific wavelength increases steeply. Accordingly, when this film is applied to semiconductor laser chips with the scattering of wavelength that are located in a row in one bar, it is difficult to obtain a reflectivity of 1% or less with all of the chips of one bar. The same results as above will be also obtained when the error in the film thickness in the manufacture of the film is about or more than .+-.10%.
It is also difficult to obtain the desired refractive index with the regulation with conventional dielectric films used in optical products such as semiconductor laser devices. In particular, when laserchip bars or the like are coated with the dielectric films, because there is scattering of the oscillatoon wavelength in each bar, it is difficult to have the semiconductor laser devices have the same refractive index over all of the bars (i.e., over a wide range of wavelengths).